There are a variety of alternative energy sources that could be used in aviation--in fact, some are already in use with existing technology. Among these alternative energy sources, Fischer-Tropsch fuels, bio-fuels, and hydrogen fuels are most notable. While there are some that are not practicable with modern technology, there are also fuels that are too contentious to put into use.
Most planes use kerosene, also called paraffin, to fly. Kerosene is a hydrocarbon that is derived from petroleum. Because the world's supply of fossil fuels is limited, and because some countries are not rich in fossil fuels, companies and scientists have been researching possible alternative energy sources for fossil fuels. On top of that, when fossil fuels are burned, they release carbon dioxide, which is a greenhouse gas. Aviation sources are also responsible for about 2 percent of the world's carbon dioxide emissions, so many alternative energy sources have focused on replacement fuels that will reduce carbon dioxide emissions.
The Fischer-Tropsch process was invented in Germany in the 1920s and 1930s. It works by converting gaseous substances into liquid synthetic fuels from products such as coal. This reaction begins with a mixture of hydrogen and carbon monoxide, which is combined with cobalt, iron, nickel, or to a lesser extent other metals, which creates a synthetic liquid hydrocarbon that is virtually identical, chemically, to conventional fuel.
Because it is virtually identical to conventional fuel, it can be deployed without changing aviation infrastructure. In fact, this is the most widely utilized alternative energy source for aviation, as it can be used in exactly the same way as conventional fuels.
When burned for fuel, Fischer-Tropsch fuels do release carbon dioxide, but release almost no sulfur dioxide. Currently, there are research initiatives to produce this imitation fossil fuel using renewable energy. So, although this may not help with all aspects of global warming, it is an alternative to non-renewable fossil fuels and will cut down on certain sulfur-based emissions.
Bio-fuels are fuels derived from plants such as algae. Bio-fuels can either be created from fermenting the plants and creating ethanol, or by heating vegetable oils that can then be burned in engines. Ethanol is not used with aviation because it is too unstable. However, the second process--heating vegetable oils that are then burned in engines--has been used in aviation.
The oils are obtained from plants that are very rich in vegetable oils or oils from algae, which are also high in oils. Although they could be used alone, the most common way of using bio-fuels is to mix a bio-fuel with a traditional fuel. There are problems, though, with using bio-fuels at high altitudes because they do not function as well at low temperatures as traditional kerosene. For this reason, bio-fuels are still being researched; though planes could fly lower or use different mixtures of bio-fuels to combat this problem.
Airplanes have already flown using hydrogen as their energy source. However, at present time, hydrogen is not a practicable alternative. It would require dramatic changes in the shape and design of airplanes because hydrogen takes up over four times the volume of conventional kerosene. However, hydrogen also weighs less than traditional sources, so design changes, while still in research, are certainly possible.
There are proponents and detractors of hydrogen energy, as some believe that the water vapor released at high altitudes is more dangerous than carbon dioxide, while others believe carbon dioxide is less dangerous. However, that could be remedied by having planes fly lower.
Alternative Energy Sources That Don't Work
Given existing technology, there are several alternative energy sources that do not work in the aviation industry. Two of those are methanol and ethanol, which are alcohols that are the product of many natural chemical processes. They are too volatile to work with existing technology, and contain too little energy to allow a plane to go beyond a very short distance. Bio-methane is also unlikely to work, given the need for jet fuel to be consistent and uniform in composition. It would also be difficult to obtain enough bio-methane for this process to be practicable.
Politically Contentious Alternative Energy Sources
Nuclear energy could theoretically be used to power aviation. However, this is very contentious and there is little interest in pursuing this technology at present.